At present, for energy saving and reduction of working noise and cost, permanent magnet synchronous motors adopting a sensorless field oriented control (FOC) policy are massively applied to outdoor fans of air conditioners. Since the outdoor fan of the air conditioner has initial starting working conditions such as pneumatic forward direction rotation (namely forward direction rotation under external wind action), pneumatic counter rotation (namely counter rotation under external wind action) and stillness, in order to improve starting reliability of a motor of the outdoor fan, the motor needs to have a certain starting capability of overcoming the external wind action. For a three-phase permanent magnet synchronous motor suitable for the sensorless FOC policy, a counter emf of the three-phase permanent magnet synchronous motor is of a sinusoidal waveform and contains rotor position information of the motor, thus a rotor position angle and a rotating speed signal can be obtained by detecting the counter emf, and whether the initial starting working condition of the motor is the pneumatic forward direction rotation or the pneumatic counter rotation is determined according to the plus or minus characteristic of the rotating speed signal, so as to determine different starting modes. A motor starting implementation process is as shown in FIG. 1. As regards an existing motor starting control technology, as can be learnt from FIG. 1, no matter what working condition (pneumatic forward direction rotation, stillness or pneumatic counter rotation) is adopted to start the motor, a bootstrap capacitor in a motor driver needs to be charged prior to execution of the steps of performing positioning control over the motor, controlling the motor to perform accelerated rotation in a vector control mode, controlling the motor to enter into a conventional operation state in a vector control mode, etc., and the concrete reasons are described in detail as follows:
A topological structure of a driver of a typical three-phase permanent magnet synchronous motor (PMSM) is as shown in FIG. 2. In practical application, 6 switching tubes and 6 freewheel diodes in FIG. 2 are usually integrated in an intelligent power module (IPM). For simplicity of design, when the IPM is applied to the outdoor fan of the air conditioner, a driving circuit in the IPM is usually of a single power supply control solution; when the single power supply control solution is used, it needs to be guaranteed that a control power supply can provide a correct gate bias voltage for an upper bridge arm switching tube reliably, and a high voltage passing a direct current bus does not flee to a control power supply circuit to burn down components. In the prior art, a sampling bootstrap circuit is one of methods for implementing the aforementioned single power supply control solution. As shown in FIG. 3, the sampling bootstrap circuit comprises a bootstrap resistor RBS, a bootstrap diode DBS and a bootstrap capacitor CBS. When a lower bridge arm switching tube Kd is connected, a power supply VCC charges the bootstrap capacitor CBS through the bootstrap resistor RBS and the bootstrap diode DBS, and a charging loop is as shown in an imaginary line loop as shown in FIG. 3. In a charging process, in order to alleviate pressure over the control power supply VCC, the connection of the lower bridge arm switching tube Kd is not continuous and is performed according to a certain duty ratio, and an upper bridge arm switching tube Ku is cut off in the whole course; when the upper bridge arm switching tube Ku is connected, the bootstrap diode DBS performs reverse cut-off to isolate a direct current bus voltage from the power supply VCC, so as to prevent the direct current bus voltage from fleeing into the control power supply circuit and burning down the components.
In sum, corresponding starting operations need to be executed on the motor after the bootstrap capacitor in the motor driver is charged, and at this time the initial starting working condition of the motor possibly differs from the working condition detected before the bootstrap capacitor is charged, so that it leads to motor starting failure very easily, and the motor starting success rate is reduced.